It is usually the case that substance dispensing devices of the foregoing kind can be used on several occasions before the quantity of the substance stored in the device is exhausted. It is also a common requirement that an accurately metered amount of the substance is dispensed each time the device is operated.
Devices of the foregoing kind have been provided with means that enable the user to determine when the stored quantity of the substance is getting low. Various different arrangements have been provided for that purpose, but they have not been entirely satisfactory because of cost, lack of accuracy, or difficulty to read or interpret.
Another problem encountered with devices of the foregoing kind is the possibility of the device dispensing less than the intended quantity of the substance. That may occur because of inadequate care on the part of the operator, and may also occur because the stored quantity of the substance remaining in the device is too small at the time the device is operated.
It is an object of the present invention to provide a relatively inexpensive, accurate and easy to read usage indicator. It is a further object of the present invention to provide a substance dispensing device including such an indicator. Still another object of the invention is to provide a substance dispensing device having a usage indicator, and also having means preventing operation of the device when the quantity of the stored substance approaches exhaustion.